Here’s why IVF is wrong (a rare instance where the conciliar bishops nail it):

https://www.usccb.org/issues-and-action/human-life-and-dignity/reproductive-technology/begotten-not-made-a-catholic-view-of-reproductive-technology

But the question is whether one can attend the baby shower of a child conceived in such a way (ie., is it akin to attending an immoral/invalid wedding reception, whereby the couple will construe your presence as support for the immoral act).

I would go and gift the baby with a blessed crucifix for the wall in the nursery. 

If the couple having the baby shower is married, then I don't see a problem attending.  Yes, IVF is wrong, but that's their sin, not yours.  I think the principle is that we avoid situations where our presence would support an ongoing sinful lifestyle.  That is, we would avoid attendance at an unmarried or divorced couple's shower.  But IVF is more of a one-time sin, not necessarily a lifestyle choice that is anti-catholic.

If a catholic couple separated and the wife had an affair and got pregnant, but then the couple got back together, and amended their ways, you could go to the baby shower.  They made a mistake and it's not the child's fault.  I don't see how an IVF baby makes a couple living in sin.  The only issue would be if the couple openly supports IVF and this baby shower is advertising the good effects of this sin.  But that's probably not the case.

OP here: In this case, the couple was very open about being excited to try IVF at family functions.  Almost certainly it will come up at the shower.  They are not religious people, and would not view IVF as sinful or a mistake, and I’m guessing if I told them the Church’s stance on it, they’d scoff.

You’re going to anticipate the birth of the baby, not the method by which it was conceived!  Unless you’re going to say the baby doesn’t have a soul?  If a woman was raped but decided to keep the baby and raise it, would you refuse to attend the shower?  How do you really know the circuмstances under which a baby was conceived unless you were there?  A married couple could engage in impure acts in the process and not say anything.  None of this is the baby’s fault.  He’ll be born with the same Original Sin as anyone else, not more if under sinful circuмstances, not less if his parents were very holy.  
I do think it’s in poor taste for a couple to announce the details of the baby’s conception, but again, it’s the couple’s sin, not the coming arrival.  I once went to a shower for which the baby was not present.  It was an arranged adoption where the birth mother, the pregnant woman, was not present by choice due to her particular circuмstances which the adopting couple did not reveal. The gifts were mainly for the baby, not the adoptive couple. Without the baby, who would give a couple in their late thirties gifts of onsies, bottles, diapers, a diaper bag, crib sheets, receiving blankets, bibs, tiny clothes, a home knit bunting and matching booties, a carriage-stroller, rattles, crib mobile, a cuddly stuffed lamb, powder, baby wash, baby tub…
Go, and celebrate a new soul soon to arrive in this world!  

Well, while it was an isolated action and doesn't have an ongoing effect of the child, you have to be careful to avoid giving the appearances of approving, and I would feel that attending such a function and saying "congratulations" would be tantamount to condoning/approving of how the child got there.  I've avoided going to baby showers (causing some family discomfort) where the baby was illegitimate.  We let the couple know that they needed to get married and that we can't approve of the situation, and then mailed them some diapers or something in lieu of attending the shower.  And the rest of the Conciliar "sheep" "Catholics" just went there approving of the entire situation, etc.  Had the entire family refused to show up, that would have sent a strong message, letting them know they're ostracized from the entire extended family ... and that would exert some pressure on them to actually get married.

By  your attendence at the shower you are publically  condoning  IVF even if you privately disapprove of it.
Do they even intend to have the baby baptized?

Well, while it was an isolated action and doesn't have an ongoing effect of the child, you have to be careful to avoid giving the appearances of approving, and I would feel that attending such a function and saying "congratulations" would be tantamount to condoning/approving of how the child got there.  I've avoided going to baby showers (causing some family discomfort) where the baby was illegitimate.  We let the couple know that they needed to get married and that we can't approve of the situation, and then mailed them some diapers or something in lieu of attending the shower.  And the rest of the Conciliar "sheep" "Catholics" just went there approving of the entire situation, etc.  Had the entire family refused to show up, that would have sent a strong message, letting them know they're ostracized from the entire extended family ... and that would exert some pressure on them to actually get married.

This.  I wouldn't go.  In general.  We need to quit putting human respect above living the cross of being Catholic.