I'll start by saying it's generally a bad idea to try to shorten the landing interview by bending the truth or making something up. Time and again people come up with ways to get around an awkward situation and that'll only sink your friend faster. As noted in the comments, your friend will be questioned, the only variable is how much. As a matter of opinion (and of a recent question on this site) I would also advise against assuming that he'll be granted entry painlessly because of his citizenship.
I expect he'll be grilled about these topics.
It's the border official's job to be suspicious, your friend has no premise for being there. A recently unemployed person suddenly decides to stay in the UK for a few months ? Seems unlikely.
Given that he's unemployed, he'll need convincing evidence of solid ties. Something like property ownership, upcoming medical appointment or family. They want to know he intends to leave, and as you've noticed, a flight to mainland Europe is easy enough to dispose of so as not to count very much. This answer could be helpful reading about home ties.
He has savings, but he'll need to prove he can cover his entire stay there and his return ticket, seeing as he doesn't have one. Note that according to this answer having a credit card might not be enough. Although the answer in the link talks about visas (which he doesn't need) I think they're a good guideline to proving his spending pattern won't ruin him while in the UK.
They'll also ask where the money came from, this question talks about that.
Relationship to you
Even though you are not in a relationship, the burden of proof is on him. The length of the stay will look suspicious and he'll need to convince the border official he's not trying to move permanently. They will ask details about how you met, how long you know each other etc. The fact that he has no previous travel history will not help him because these are sometimes used to establish patterns of travel.
Even though US nationals aren't required to get entry clearance before arriving to the UK, doing so might simplify his entry once at the border. In the case of refusal, at least he'll avoid the hassle (and possible removal) you don't want him to experience. Read here for how to apply.