He is wrong. As long as you complete your journey by about 5am on the day after the ticket runs out, you can stop as many times as you want.
So say on a SVR, Brighton to Victoria. You could on eg the 1st. Go Brighton to Victoria (after the 8.45 has left), then on the same day go Victoria to Gatwick. Fly away on holiday, then on the 1st of the next month catch the 04.42 from Gatwick (04.00 from Victoria) and get back to Brighton, and your ticket would still be valid for that service.
Only thing of note would be to keep your ticket safe, and if it gets marked by the guard on your return from Gatwick, dont look miffed off. Handing over a scrunched up, worn out ticket and trying to avoid getting the guard to mark it sets off alarm bells.