LAMP
Should, coulda, woulda. The FBI "shoulda" questioned the tipster (1) for the names of "everybody" at the party, (2) how long was she forced to remain at the party (3) was she ever invited or coerced in any way to participate in the meme and (4) what did she witness herself.
The information is too sketchy to determine what the FBI did . . . or didn't do . . with the tip.
On the dark side, if the tipster's claim about a "party for prostitutes," was true it sounds like Trump was helping his good friend, Jeffrey Epstein, "audition" future candidates for his sex trafficking enterprise for his stable of VIPs.
It would also answer the question of . . . How much did Trump know and when did he know it.
In short, Was Trump both a user and a facilitator and (possibly) did he get first pick
What DOJ prosecutor or FBI agent would want to slip their neck in that noose. (Breaking Trump's transparency law.)
Best to punch the time clock and try not to get caught leaving by the front door. (Use the janitors' entrance.)