??? How Do You Determine How Far You Can Take An Amp When Choosing Speakers ???

Money is always an issue but not the key issue here.

You always want an amp capable of driving specific speakers to their max without having to break a sweat for best performance/objective results.   

Beefier amps will do it with more speakers.   Less efficient speakers will work best with fewer amps.

Note:  What works best from a technical/engineering perspective is not the same as what sounds best to a particular listener.   What someone thinks sounds best is a purely subjective determination.

"Weakest link" and "source first" philosophies (etc) belie the nuanced complexities of human perception and system building. Though when you’re starting out, it can help to establish a sense of order amongst the chaos.

You can have great successes and failures with any of these combinations:

• Expensive amp, cheap speakers
• Cheap amp, expensive speakers
• Expensive amp & speakers

By "cheap" I don’t mean junk, and I’m including used-market bargains. Finding a beloved cheap amp & cheap speakers setup is also possible, but for most of us here this is playing the game in "hell" mode difficulty - same with buying only new at MSRP. 

Like any nontrivial hobby this is a journey, and a young man like your friend’s son will have to find his own legs, then start forging his preferred path. Since he’s a capable EE, he might look into vintage amps. I think also it’s important to hear several different speakers brands & configurations, to start shaping a sense of what you like. I honed in on Tannoy as my favorite speakers, and since then I’ve grown quite fond of pairing them to "extravagant tube amps" - but vintage tube amps are still fun too, and modern solid state (even Class D) can sound good too! Tell him to go forth and explore!

The audio industry has always promoted over spending on loudspeakers. Why? To keep the Audiophile on a never ending upgrade quest. Simple formula a budget $2k loudspeaker with a $20k amplifier will out perform a $20k loudspeaker with a budget $2k amplifier in almost every circumstance.

@freediver

Sounds like your friend may have a lot of gear around? Including some hard to drive speakers perhaps? 

If so, you may want to advise his EE son to grab his oscilloscope and see how soon the Peachtree starts clipping when driving the hard to drive speakers. Depending on what he sees, he may decide to start considering more capable amps.

Wow, that is a challenging question. It’s a bit easier if thinking about the beginning of the audio chain, since if you have a poor signal, better components may make it sound worse. But at the speaker end, unless you are under powering the amp, generally, the better the speaker the better it will sound... I guess unless you are getting a tremendously detailed speaker... for example Magico... too revealing. I have heard Magico make some pretty good equipment sound bad. Hmm, thinking out loud... obviously. 

Most of us are trying to set ourselves up for the next upgrade. But sounds like he does not want to. So, I’d go for the 2x to 3x area. If he doesn't know he is going to upgrade in the future... then being conservative makes sense.