Understand the reasons for him to hit on you. If these reasons are purely physiological then explained it to him, that he made a choice for your best friend and it is unacceptable for a man to fall meekly under lust. If there are other reasons, then ask him to be precise in explanation. After that evaluate what he says and apply it into your situation. Answer your self: what is the purpose for all of this to happen? Should it happen the way he proposes? Did it happen to me before, where scenario of events was the same but with different people?
If you consciously be able to reflect upon this matter, I am sure your answer will pop up out of no where and words will be precise. And the fun part is, there will be no thinking process involved ;)
Do not Stress and Good Luck! :)
p.s. when the problem arises for you, it means you already know the answer, all you need is to get to this answer!

better to ignore or keep silence.

situation gives you a bitter taste if you had shown any angry.

Just ignore cause the more you say something the more they hits you... just keep it silent or gone without knowing them ;)

Be frank with him, set boundaries and make sure you maintain those boundaries. To me, if you me, i'll push harder

Tell him you'll report it to your friend if he ever repeats it again. And if he tries it again, report it immediate cos he may lie against you to your friend before you report him and seperate your friendship.