The kids don't tend to pop pills nowadays as MDMA is seen as stronger and more pure (still basically the same stuff though). If he's got a job and a normal sort of life routine I won't worry too much at this stage. I guarantee it's not the type of drug you can do on a weekday night and hold down any sort of responsibility the next day. It's also not addictive like coke is or anywhere near as expensive which means it's less likely to ruin lives in the long run.
After a few small lines of MDMA he'll have been awake all night and have had zero sleep. About 12 hours later he won't have felt great (sweaty, feeling sick, hungry but not really able to eat or sleep).
You said his girlfriends mum picked them up and this is a good indication that they only took a very small amount. If he was a regular user of it or had took a larger quantity it's all most certain they they wouldn't have been able to hold a conversation or even a turned up for a lift - it would be way more obvious than diluted pupils.
My guess is that it was a first time type of thing because you'd definitely not want to sit in a car with someone's mum if you knew what you'd be feeling like.
My advice would be keep an eye on it and take notice if he changes his group of friends or starts staying out all night regularly.