Attack is the best defense
It looks like you are afraid that he will somehow make your thesis his own, maybe defend or publish it before you do, and then it looks like you plagiarized his work.
If that is your fear, then I can somewhat understand your concern. I assume that, so far, nobody but you and your advisor have seen your work, so it would in theory be possible for that scenario to occur, although your advisor should hopefully remember that you came up with it first...
One way to get out of that conundrum is to make your work as public as you can. Put it on some personal website, or your personal (public) university page if you have one. Send it to a mailing list, if you have one that is fitting for the purpose, say a interest group at your uni or wherever (which has the benefit of having integral timestamps in their archives). Post it to the website of some interest group that is closely related. This will make pretty sure that people know that you were "first".
After that, just send your friend a link, and enjoy discussions about the topic. He will be welcome to do research alongside your topics, if they interest him, and it is pretty certain that a) he will not get any bad ideas since your work is publicly related to you already, even if you have not yet defended or published it through traditional means; and b) if he does turn bad, then it will be easy for you to come up with proof that you were first.
"I may do future work based on my thesis submission"
That's great, and if your friend does end up doing similar work it should not really be a problem, right? If you have the feeling that there is a "gold nugget" there, some secret line of work which is easy for you to do while still getting full credit later on, then that is a fallacy. If the topic is so shallow that one more person working on it would "spoil" it, then that is not a good base to build your future on. Besides, it might just turn out that your nugget does not work out, and what then?