Oh Man, Day Home mamma: TAKE THE HIGH ROAD! My feelings would be hurt but, don't forget why she took him out...not because she is upset with your care, she obviously loves you, but if someone bit my kid (and twice?) I would remove them from care too. Seeing that Friday was to be his last day?...I may have left him in for the week, BUT how do you know he wasn't "crying" about this at home and maybe begged his Momma to "not send him back"....who's the biter? and why? thats another post in itself, but I would seriously consider letting the biter go. (I don't tolerate biting, only had one in 10 years and let him go at first offense )
If I were you, I would call the Mom, apoligize one more time and ask her if she could please bring son over to have a proper good bye, (maybe pick a day the biter is not scheduled) Don't burn a bridge with such a good friend and child.

I regret the way one of mine left, after caring for 4 years, he left 2 weeks early due to a new "habit" of puking every day all over my lunch table. (he had emotional issues, and OCD) so when he got in the "habit" it was hard to break it. Well...the parents got tired of being called from work about 3 times a week to pick him up, so they pulled him 2 weeks early. (some thanks to me for care for 4 years) I did realize they were doing it for other reasons than being "angry" with me. 1)they didn't want to leave work anymore
2) they had to pay for the day even though he puked at 8 am
3)they didn't think it was good for him to keep his "habit"

this Mom could just be looking out for her sons best interest, nothing personal.....just reach out to her and him, you won't regret it!
dont forget too....this transition is stressful for her already, then add the bites (put yourself in her shoes)