Hey matebaric69!
No need to apologize, I totally get what you're describing, and I think I have an idea as to why you're missing this skin.
If you're seeing that Pool Party Taric was removed, its not usually due to an error regarding the account. Typically, this content is removed due an instance such as a player purchases RP and gifts content with it, and subsequently issues a chargeback for the money spent on that RP, we will remove that content.
Basically, your friend may have disputed the charge for the RP he used to get you the skin, and got a refund. If that's the case, Pool Party Taric was then no longer paid for, which is why the content is no longer on your account.
If you believe that content you have received on your account has gone missing and is not due to what I've described above, make sure to submit a ticket! We can look into your account and investigate further!