So did you have him when he used to nap? Or did he come to you, as a non-napper? My policy is all children under 3 have an afternoon nap, and the others have quiet time. This is when we can put on a movie or some music, we can read or do puzzles, play a quiet game, etc. The bottom line is....shhhhhh. The kids know that I will not be playing "with them". I use this time to get things done, or just have a little bit of time to relax.
I agree that if this is the first time that he has fallen asleep on his own, he did pretty well all things considered. You mentioned; "the heart breaking hurt this child seems to feel without sleeping with mom". As much as I am okay with co-sleeping arrangements, it is still necessary for a child to learn to sleep by themselves.
I would recommend that you speak with mom and dad, though. If he is a full-timer, then it's important that they know what your day-to-day is. Otherwise, if you continue to put him down for naps when they are under the impression that you are not, in my mind it's dishonest. Explain to them that being at daycare is very stimulating both mentally and physically for a toddler, and that by mid-afternoon thier little angel is beyond ready for a rest, both mentally and physically. Ask about bedtime, and it's routine. If mom and dad are putting him down at 6:30 or 7, because they are probably tired, then he will definitely give them trouble at bedtime.
As for continuing to cry after nap; it sounds to me like he is still tired! Hard to say, but I would keep it up. In order to strike a compromise, perhaps you can put him down to sleep last and wake him up first?