How do I confront this situation

So I'm with my boyfriend this weekend and we went to his friends house and they had like a game night and stayed up until the sun rose and I stayed up with them watching movies. We left at 9 am because his best friend had to go home and watch his 2 year old while his wife went and ran errands. We get here and my boyfriend and his friend pass out on the couch after his wife leaves and I'm like well I'm going to watch him of course. It was no big deal we ate and then we played trucks and read a book and watched a movie and he got his day outfit on and we had fun. I love children I had no problem with this at all even though I was exhausted from not sleeping last night. Well eventually mom comes home and sees that he's still asleep on the couch and she's pissed. She said "I'm so sorry you had to watch him, thank you so much I'm glad you're here. What if you weren't here he would have just left our kid completely unsupervised" and I was like.. Yeah it's not a big deal. Well dad wakes up from his nap pissy because she was saying all of that and he got an attitude with both of us... Like..