My [26F] husband's [31M] best friend [31M] behaved inappropriately. Do I need to tell my husband?

My husband (of five years) and I have a good relationship. I have no major complaints; he's a good man and husband. I love him a lot and feel pretty torn about this incident as I don't want to upset him (with negative information about his friend) or jeopardize our marriage if he doesn't believe me. But I can't stop thinking about it. I'm replaying this over and over in my head and actually losing sleep. I need to talk about it with someone but I'm not sure my husband needs to know.
Last weekend we had some friends over to hang out. Among them was my husband's best friend (of 25+ years) and his girlfriend. They both drank too much and it was decided they'd spend the night at our house. Sometime around midnight my husband went up to bed. His friend went off to the guest room shortly after where he fell asleep. It was just his gf and I in the living room for awhile and eventually we both fell asleep on the couch. It's important to note we have a large sectional and she and I were on different parts of the couch (not close at all).
I later woke up to someone touching me on the couch. I felt a hand on my inner thigh, lightly squeezing then sliding up to my crotch area (over my pants). I thought it was my husband and giggled at first. I opened my eyes and rolled over to discover it was not my husband but his best friend. I was mortified and kind of scared. He jumped up and put his finger over his lips. I sat up and started asking what was going on. He said he thought I was his gf and apologized. He quickly returned to the guest room and I went up to bed.
I keep thinking about it and I really want to believe him but I just don't. He didn't seem that startled when he "realized" it was me. He was oddly calm the whole time. I also don't look like his gf at all. I'm like 7" shorter and we have different hair colors too. I'm having a hard time accepting this was a mistake, even if he was still drunk.
He messaged me on Facebook earlier this week, apologizing and explaining he thought I was his gf (again). It was a carefully worded and short (not details about the incident). He didn't ask about my husband but I suspect he was trying to figure out if I mentioned it. I didn't reply and he hasn't contacted me again. I find the message a little suspicious because my husband doesn't have FB and he has my phone number (and he's texted before). I also think it's somewhat telling that he hasn't messaged me again or attempted to call or text about it.
I've considered his side too. Perhaps it was a mistake and he's really embarrassed? It is possible my perception was off having been sound asleep at the start of this incident.
I don't know what to do. Should I reach out to his friend for a better explanation? I'm afraid telling my husband will devastate him and possibly end his friendship. It's also possible he won't believe me or he might think I'm blowing it out of proportion. If so, it will obviously cause problems between us and really don't want that.

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tl;dr: My husband's friend touched my inner thigh and crotch over my pants while I was asleep on the couch. He claims it was an accident and thought I was his girlfriend. I don't believe him and I'm not sure what to do next. Do I need to tell my husband?


Source.

He was drunk and picked the wrong girl lol. Would take some big balls to purposely grope the OP while his GF is on the same couch.

But hey, I see nothing wrong with telling the husband either.
As a matter of fact, if it was me, the next morning, I woulda told my bud...."Hey man, I was drunk and groped your wife thinking she was my GF, my bad. Honey, get over here so he can get a handful and we'll call it even"

I think it's a lil to late to tell the husband now. You should have told him the next morning. And you should have replied to his friends message, you are making it awkward between you two and the husband is gonna suspect something when his buddy stops coming around. As for if it was on purpose, probably not, he was drunk and half asleep...things happen.