If POTUS has solid evidence and has given it to Biden, Biden should concede rather than risk knowingly taking part in a fraudulent election, AFIAC.
That would be, for example, if the electoral votes that put him over 270 are shown validly to be fraudulent.
If a saste withdraws it electors and Biden doesn't have more than 270, it would be different, unless electors moved to Trump to put him over 270.
Biden needs an out, though. It could be that POTUS's call to GA on Saturday, telling Raffensperger to find/verify fraudulent votes was to give Biden an out.
Biden could be given a parachute by virtue of the claim, which would be agreed to by Trump, that Biden wasn't in on it, but would concede, "For the good of the country".
GA didn't play ball, maybe due to today's election. Other states may be up to bat soon.