The route is to finish a PHD first, then find a lecturer job/ or post doctor research fellow position. The starting point is around £30k. You can search the salary scale for university for this.
This is optimistic to the point of being unrealistic.
I'd suggest he applies for an MPhil (or similar). There's no harm in that and it will give him a sense of whether or not he enjoys further study. Lots of good people go on from that to train as lawyers anyway. The best person in my MPhil cohort did.
After than, if he wanted to continue, he'd apply for a PhD/DPhil. This is a longer commitment of time. Again, nothing to stop someone with a PhD changing tack and going into law afterwards.
After a PhD, he'd apply for postdocs. It is vanishingly rare to go straight into a lectureship. Postdocs do not typically start at 30k. Indeed, Oxford offers a lot of 'stipendiary' postdocs for 3k and upwards. These are designed to provide funding (and, arguably, exploitative teaching) for people who're just finishing or have just finished their PhDs. Another popular and exploitative option post-PhD is the nine-month teaching contract, where you are not funded over the summer (when you'd do research), but you are expected to be 'research active'.
You might do postdocs for five or more years. So, in your early-to-mid 30s, you might be a realistic possibility for a lectureship. That would start at 30k, these days.
The numbers of people who make it that far are very, very small. A lot of people drop away, and often money is the issue. Moving every couple of years and having lots of financial uncertainty is quite difficult.
(Incidentally, a professor is a very senior academic - it's only in the US that 'professor' is the general term for someone teaching at university level.)
I am not saying this to put him off. I'm a postdoc; my subject is fairly similar to Classics in terms of employability/career paths. I love it and I don't regret doing it at all. But a lot of people feel upset that no one really told them how hard and uncertain it would be. There's absolutely no guarantee you'd get an academic job. I know people who are excellent - first class degree, distinction at masters, excellent PhD, two books published, prestigious postdoc fellowships ... and they've still not ended up with permanent jobs in their mid 30s.