Inappropriate behavior between 4 and 5 year old boys no

My son is 4 his best friend is 5 and Earlier today when I was watching my son (4) and his best friend (5 yo boy), they were caught with their Pants down (literally) in my son's room. They were talking about licking each others butt and apparently penis and we're tickeling each other there. Is that completely abnormal for their age? I don't know where they got this kinda behavior from. My son is not exposed to sexual behavior at all. I am careful about what's shown on tv, on youtube and he goes to school online with me right there. In fact I'm a victim advocate and talk to my son about boundaries, appropriate touch, and body autonomy all the time. My son said it wasn't the first time they did "naked stuff", which is quite alarming to me. Please tell me what I can do. I'm a little shaken up.

Of course I spoke with 5yos mom and we talked to the boys individually. I also know they're safe at her house and haven't been exposed to sexual stuff there. She runs an in home daycare and her spouse is military.

My son said it was her sons idea too...so help with talking to her about it would be helpful

And no judgment please

Update:

Spoke to his pediatrician and the behavior specialist there. They agreed that it was a normal behavior for him and the other boy as they're both about the same age and very close. They said there are no red flags (using language that they wouldn't know, using "poses" or mimicking acts, or trying these things with adults or older kids. They said that they were exploring and it wasn't coerced (like it was playing/exploring and not pressured on either part). They didn't think it was abuse or anything, but said for us parents to just keep talking about boundaries and keeping clothes on when with friends. They said we handled it well too.