It can be done, but as Romelle says, if your friend does not read Russian then she had better have a very solid plan in advance, with guidance on exactly where she's going and how those names look in Cyrillic.

Absolutely do not use cabs. Driving between SVO and the center will eat up most of the 7-8 hours. Moscow traffic is unbelievably nightmarish these days. The Aeroexpress/metro is your friend.

The whole visa question is another big complication. I don't know the difference in documentation/cost for a transit visa vs. a normal tourist visa, but the tourist visa is enough of a PITA and expense that I wouldn't bother with it just to see Red Square. If a transit visa is cheaper and simpler, it could be worthwhile.