So the whole of Buffy upto OMWF boiled down to its essence in four episodes. Not easy 🙂 But here's my attempt . . .
1. Becoming (both parts, but if not, Pt 2) This episode shows who Buffy really is, the girl and the slayer. It speaks to her history, what she's had to do in her past and the horrors she's seen and perpetrated, and of the depths of love she feels. It shows the scoobies both working together and slightly at odds, which is their usual mo, and Buffy's home life and relationship with her mother. There is a snapshot of Spike, pre-chip but still making a truce with Buffy. I think it gives a good first view of all the major players in Buffy's life so your friend will know who everyone is in OMWF
2. Helpless - (OK you might have to explain why Angel is back from the dead) This episode is about the complicated relationship between Buffy and Giles and how Buffy sees Giles as her father figure. I think the scenes between them in OMWF are so emotional, but only if you understand their relationship. Also, even without her supernatural slayer powers, Buffy Summers is still an amazingly resourceful girl, and has strength that even she didn't know she had. Knowing this , and then seeing her suicidal in OMWF, is heartbreaking
3. Family - This goes some way to explaining who Dawn is what's she's like as a person and her importance to Buffy and she show. It shows Xander and Anya are together, and the relationship between Willow and Tara, and that they are witches. It has all the scoobies standing together against a common enemy, and Spike is now scooby-adjacent and helping them out.
4. The Gift - Buffy's sacrifice, the reason she died, which I think you do need to know to make OMWF make sense. It also has Xander proposing to Anya (their wedding being a big plot point in OMWF), some lovely Spuffy which makes the kiss at the end of OMWF more understandable, Giles kills Ben so showing he takes a lot of Buffy's tougher jobs from her, Tara getting her mind back which sort of explains why Willow's memory spell was so terrible.
I think those 4 eps explain who everyone is and where they are at by OMWF so making that episode easier for your friend to understand. And once she's seen all 5 eps there would be no way she wouldn't want to watch the whole 7 seasons! 😀