My male fiance has had no sexual desire since starting on 10mg of Lexapro and 150mg of wellbutrin (bupropion). Would horny goat weed or tongkat ali help at all?

Reduced sexual desir. I agree with dr. Wartenburg on the need to be careful with herbs here in the us, and speaking with your husband's physician before adding any of these. There are no interactions between horny goat weed, tongkat ali (actually eurycoma longifolia), lexapro, and wellbutrin (bupropion). There is also not sufficient research data to assure that either of these herbs will work for the intended purpose.
Hard to say. It is common for Lexapro (escitalopram) and other ssri's to reduce libido and sometimes sexual function, but bupropion can often reduce or even reverse that effect. I am not a big fan of most herbal remedies, largely because they are not regulated in the us as they are in germany, and you can never be sure of what you are getting. I suggest he talk with the prescribing physician about this.