Health Warning:
Much of the "go for it", "follow your heart", "don't listen to the naysayers" type of advice comes from wannabes with much enthusiasm but no commercial flying experience. Much of the 'negative' advice (realistic, if you prefer) comes from people who have been in the industry a few years and are pretty familiar with the ways of pilot recruitment. Your friend would be well advised to bear that in mind.
To answer your questions:
(1) Age is less of an issue than it was, but 46 is pushing it a bit, as he would be at least 48 before he qualified. Not impossible, particularly when times are good (it's not unknown for people to get into the industry in their 50s) but it will be a challenge.
(2) Nope, £28k is not nearly enough. Zero hours to CPL/IR (the basic qualification to fly commercially) would cost a minimum of around £40-45k via the modular route, or £65k+ with integrated training. Many airlines (not least Ryanair) also require new entrants to pay for their initial type rating, so you can add another £20k or so to that total.
(3) Very, very few jobs for inexperienced pilots at the moment, with many hundreds of people chasing them (I guess that makes me sound negative, but reality can be a kicker sometimes). The market will improve eventually, but probably not in any significant way for at least a couple of years.
(4) As a flying instructor, £10-15k, as a turboprop FO, low to mid twenties, as a jet FO, £40k+ (but that kind of starting salary would probably require paying up front for a type rating). It's not a lot considering the cost of training, particularly if you only have a 15 year career to pay back the investment.
Frankly, given your friend's age, the state of the job market and the amount of cash coming his way, I would be looking to do a PPL and have some fun flying privately. To me, the potential benefits of him changing career right now would be outweighed by the risks.