I just recently found out that I have lost a skin but I do not know if I lost any other skins, I had two udyr skins, Spirit Guard Udyr and Black Belt Udyr. The Black Belt was gifted from a friend, I was going to play Udyr and Try Black Belt because I have never seen it before. I saw that I have l lost it when I saw it was on sale and it didn't say owned, I tested it by going into Champion Select and Saw that I only had Spirit Guard.
If your friend was subject to a chargeback or RP fraud, any content he purchased with that RP will be removed. Chargebacks are the most common cause of losing mystery gifted skins randomly, so check with your friend and see if this happened. If you can't contact him (he'll probably be suspended atm) or can't confirm this, you can submit a Support Ticket and ask them for help.
For more information about chargebacks go here; https://support.riotgames.com/hc/en-us/articles/201753334-Chargebacks
