try 'infidel' or 'pagan'. much easier.
(bad joke; haven't had my coffee yet this AM).
Seriously, though, 'Muslim' accords the person's religion too much respect. 'Mohammedan' is more correct if you are simply referring to what they believe and not trying to seem to approve of it in any way.
I know people that will only refer to Catholics as ROMAN Catholics, because that's what they are (and it means they respect the person's beliefs despite differences). I have also heard the word 'Catholic' said in such a way that it sounded dirty, disgusting and dismissive.
FWIW, 'Papist' or 'Romanist' is the Roman Catholic equivalent of 'Mohammedan'. Shows that there is a recognition of one's belief on some point, but is entirely unsympathetic to that belief.
Honestly, I just don't see the value in 'making nice' when it comes to false religions. There are myriad ways to be kind and respectful to PEOPLE without approving one bit of their pagan or heretical views.
All that to say, use 'Muslim' if you are in a situation where using 'Mohammedan' might cause a fight. Keep 'Mohammedan' in your mind, though, no matter what you're saying.