Not sure what time they want to depart.
As a starting point, a Crewe - Perth Off-Peak Return is £110.70. It's valid at any time.
http://trainscanbecheaper.info/ suggests that a split into five tickets is possible to reduce the cost of this ticket to £92.90
This isn't the most compelling of discounts and so Advance tickets are probably a better option.
The 19th at 16:09 changing at Preston and Haymarket is £35 Advance Single.
Returning on the 20th, there's a £22.50 single, at 15:37, which NRE warns uses a bus. It turns out this bus is only to cover the short walk between Glasgow Central + Queen Street, so not really a big deal.
It does however suggest that it might be worth forcing the outbound via Glasgow; this however elicits no savings.
So £57.50 return using Advances both ways, and the price will rise as the date gets nearer so do book very soon.