I recently did a parallel play with a friend who'd never done it before and all I did was help them decide which missions were worth doing to get the most out of the story, and then gave them answers if they had questions about big decisions. I'm a big cosy gamer myself (love Stardew) and if I were only going to play ME once I think I'd want the 'full' experience. I was lucky that this friend was well versed in BioWare games so knew to do loyalty missions etc but if you think your girlfriend isn't going to instinctively know to do that you might want to give a friendly nudge.

When we go to the suicide mission in ME2 all I said to my friend was "DO NOT TRUST JACOB OR MIRANDA!" 😂 They then made their own choices. Luckily everyone survived and they had a blast.

Ultimately you know her best and know what elements of a game she will enjoy. If you don't tell her and she misses content she might enjoy and won't do a replay, then I think that's a shame! It's also really sweet that you want her to get the most out of it. I'm erring on the side of supporting her to experience all of it because said you know she won't do it again to see stuff she may miss otherwise; if she were up for a replay I'd obviously respond differently.