I'm not totally surprised that the mom didn't give you the heads up on it. When DD had lice, I waited until I knew that she was 100% clear before returning to school (and informed her school so that the other kids could be checked) but didn't advertise to absolutely everybody that I knew that she had lice. I told anyone that we were in contact with the previous few days but once it was completely gone I didn't mention it to people. No sense freaking people out or embarrassing her if I wasn't concerned about her spreading it.
Hopefully the kids mom ensured that she was ACTUALLY all clear and didn't just dump a container of chemicals on her head and call it a day. I would have a peak at the kid's head yourself to see if there are still critters running around (and would then check your own children's heads if you find anything) and vacuum or bag or wash your cover.
When DD had lice, I just ran the vacuum over the car (and all our furniture every time she had any contact with it) and it was enough that they haven't returned.
This kid's mom is a total space case. She has more than once been over 40 minutes late picking up her kid from soccer practice (no- she doesn't stay for games or practices and the kid is six years old!) She has had her nanny pick my up charge over 45 minutes late from school and take them to their house instead of taking them to said class and hasn't called me to tell me a) that she would be over 45 minutes late picking up "my" child (it's $1.00 a minute over 10 minutes late- not that I'm the one paying) and b) that she would be taking "my" child to another destination!
And THEN this other nanny that they have (they have 3) puts "my" charge in an open top Jeep, in a NBB booster with no headrests in the Jeep and the seat back ends approx at the top middle of "my" charges back. AND the kids are allowed to throw their heads and hands out of the moving Jeep. I was very firm with "my" charge that she is never allowed to do that ever, but there is still that stupid back seat issue. Not my kid so there is nothing more I can do there because I have alerted her parents and they said they don't want to "cause waves" because they will be seeing that family at their elite private school for the next 8 years. How's that for safety??
And now lice.....
Oh geez, sorry that I went off on this rant here......