Dealing with this sensitive issue requires validation from both ends. Instead of searching for a sample letter, it would be prudent to call both parties and discuss in confidentiality. Only when you are certain that sexual harassment has taken place, ask the aggrieved party whether she would be open to legal recourse or if a warning letter would suffice. Issuing a warning letter may backfire later against the company if the aggrieved party (the woman) chooses to pursue legal action later on.
Delve into the legal aspects, including potential arrest and jail, with the alleged perpetrator in detail. If he agrees to apologize and she accepts, then officially let the matter rest in writing. Sexual harassment, whether by men against women or vice versa, can be interpreted in multiple ways, so it is crucial to establish the facts and record them accurately.