First of all...
Admit (to yourself)! That is a pretty stupid thing to do... with or without consent (man... asfixia is serious! if she had asthma...).
Next...
Know that every shit has consequences? Good! Prep yourself for it first.
Calm your ass down right now. Breatheeeee!

not too much...
Apologize to her mum then... it is necessary (unless your friend manages to convince her mum by herself)
Ask support from your parents... admit all! bear the beating...
The best option for you... is talk to that girl-friend of yours... convince her to apologize together with you.
If you do your homework correctly... you should have already gain support from 3 sides (yours, hers, her parents)
There is also another alternative to clear out by your own... but let me know what you think first...