The OP found a copy of said letter on her computer. The letter is on her college’s letter head, and has her signature on it, proving that it was written on her computer. Since the OP did not write it, that leaves her son and the suspect.
So either the son wrote it, the suspect wrote it, or an invisible alien broke into the house and wrote it.
There is no other use for the letter except for the suspect’s appeal.
The son would not have known what to write or how to set up the letter, had they done it on their own. So if the son wrote the letter, it was dictated by the suspect, at the very least.
We have the letter, we have information on the letter only possessed by the suspect, we have motive, and we have opportunity.
What else do you need?
Of course, it could have been the alien.
Now, if the letter was submitted to Clemson, the suspect has committed a crime, and potentially implicated the OP in that crime. The OP should take all the steps necessary to have this come to light, for her own sake.
If the suspect has not submitted the letter to Clemson, and downloaded a copy, a crime still has been committed, albeit a lesser one - a forged signature on a forged letter.
If the suspect keeps a copy, the OP is still at risk, since the suspect still has the forged letter and forged signature, which could harm the OP in the future. It is, however, up to the OP to decide how to proceed. I would demand that the suspect write a signed admission that they created the letter and downloaded it. An admission that would only be used, should that signed letter appear in a future date.
If the letter was never downloaded, there is a long discussion due between the suspect and the OP about boundaries, trust, and the consequences of the violation of these.
At this point, I, personally, would not trust the suspect if they claimed that they didn’t download the document, and require the confession in any case. But that, of course, is up to the OP.
I would treat this type of behavior as far worse than, say, stealing money. People can grab something they need when they need it desperately without thinking. The letter required the suspect to search the OP’s computer (a major violation of privacy, BTW), look for the signature, look for the letterhead, sit down, write the letter, put it together, and then download it and send it. It would have also required that the suspect make up a false story, put together false appeal, and then put it all together and submit it.
Another thing to remember - the suspect would have had to search through the OP computer. People should stop a moment and remember how much private stuff is on their computer.