a friend came back from there and says he got stung with a large
fine for not paying the toll (through his car hire company).

Serves him right for not reading the dozens of warnings (not to mention
ignoring what the car hire company said). Why, as a tourist, did he
even want to drive through central London in the day?

He said he never encountered any toll-booths or e-tag systems.

That's because they are aincent technology that requires unneccersary
manpower. Of course they catch people on false plates, but blocking the
200 entrances to London would be impractical and add to congestion even
more.

He did encounter big warning signs saying "Congestion charge 0700-0630"

The exchange rate is bad enough without getting stung with stupid
fines. That said, I think it is a great concept!

The exchange rate is bad for americans because the dollar is crumbling
faster than a really crumbly cheesecake. No doubt you'll be invading
Iran soon enough as they want to sell oil in euros, resulting in even
more dollar devaluation.