If just to proof not "gift money", they just issue you a 1099 for 100K.
Now the problem is you want that 100K as LTCG.
Well, 以他的名义投资, invest in what? For example, if invest in common stocks, then (collectively between you two) what is LT, what is ST/ordinary, will not be decided by you (heard about IRS?). You two can agree upfront about percentage of ownership, but I don't see how you decide first (preferred) 100K profit (if any) will be yours and will be categorised as LTCG (wish it's that easy).
Maybe, think about this way and consult tax CPA. Create a company, you own 100% (big if and we'll talk later). Your friend had the right (like a call option) to buy that company from you at 200K (100K is your cost, 100K is LTCG then). Wait, I think IRS will argue you are owner operator of that company, thus such (any) profit will be ordinary income. Then you hope would be, emphasize you are only passive investor (instead of active operator) by: only own a minority stake, declare you know nothing about that industry...
