I would say that - aside from Giant-Sized and Dark Phoenix Sage - nothing pre UXM-200 is necessary for your understanding of X-Men today. Will you miss some references? Yes. You'll probably be slightly puzzled to references to the early stories of Kitty Pryde, Rogue, and Magic. Otherwise , they were X-Men, they had adventures. The end.
200-280 is a little trickier, because a lot of this stuff still gets referenced. To this day I've never completely read Mutant Massacre, but that's a key story. If you never read the Siege Perilous and the following issues you'll have no idea why Psylocke is Asian rather than British and blonde. The 260-275 range wasn't exactly awesome when it comes to plotting, but it introduced Gambit , explains why Magneto and Rogue are always flirting, and X-Tinction Agenda sets up Genosha (important in Morrison and beyond).
281-350 is crossover city, and if you understand what happened in the crossovers there's not a whole lot of other use for it. Tucked in there is some character development on Emma, Jean, Psylocke, and Iceman that still gets referenced. It introduced Colossus's brother and also the decimation/transformation of the Morlocks. However, I would say that early XMv2 actually has a lot more content - Asteroid M, intro of Omega Red, re-intro of Longshot/Dazzler, Legacy Virus stuff, Rogue/Gambit flirting, etc. I think that's more worth reading. EDITED TO ADD: Eldestrisk is right, directly above - Fatal Attractions has a lot of stuff that continues to reverberate through today.
350+ in Uncanny is a crap-shoot with a few good issues, but nothing really riveting until Claremont starts to right the ship again in 444, and now we're much closer to present day. If your friend has the Magneto War issues from UXM/XM then you should read them to understand what was up with Genosha shortly pre-Morrison.
Also, you might poke around in the 71-100 range of X-Factor of you like those characters, or various points of Excalibur through 100 if you like Nightcrawler/Shadowcat. I don't know that anything truly unmissable ever happened in X-Force.
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