Easy solution, grab that secondary ML and ask him to fake date her in front of him. Also an excuse to take them on a real double date.
The funny solution, We are dating each-other and do not swing that way. Please stop flirting with my gf.
Now, The sane solution. Have your guards kick this piece of crap out. Seriously, where are these nobles guards? Why the hell is a maid being used for protection? Also, why is this guy delivering the letter himself if he was trying to hide his identity? What about his servants?
Find out all of these answers and more next.... never!