Interesting question. I'd probably distinguish between core *G benefits and Altitude-specific benefits:

*G benefits:

most benefits such as priority check-in, lounge access, priority boarding etc. can be obtained by having a certain credit card or buying the MLL Club membership (or a similar membership).

So we'd probably be looking at $500-$750 per year. Since I don't expect the *G benefits to change much anytime soon, we can use that figure for the foreseeable future.

Altitude benefits:

If you had asked two years ago, I would have replied the ability to upgrade 1-2 int'l roundtrips if there was space available on the day of departure was quite valuable - worth several thousand dollars per year if you knew what you were doing. With the systemwide introduction of Tango fares, the increasing gap between Tango and Flex fares and the introduction of co-pays this benefit has pretty much disappeared. Sure, there may still be some value for people who buy one Europe Lat flight pass per year and do three YYZ-LHR roundtrips per year, but those are outliers. What's left is the ability to upgrade 3-4 NA one-ways if you're willing to pay a ~100% premium for Flex fares. Most people would probably value each of those upgrades at a few hundred dollars max.

You could also add the value of the miles from earning a 50% miles bonus vs. just a 25%-35% miles bonus etc., but that's not much.

Since I don't expect Altitude benefits to improve and it's more likely we'll see more enhancements in the future, I'd probably value the Altitude benefits at $1k per year for the next 4 years.

So maybe $4000 + $600 x lower of (number of years you expect your friend to be around; number of years you expect AC to be around; number of years the Altitude program will exist)