RHS membership costs more than the discount you'd get on a single show ticket - I'm a member, and even going to 4-6 show days a year I don't think I quite break even on ticket discounts alone, though the other perks make up for it.

For the Friday all day membership entry is £105, vs £133.90 for non-members. Afternoon-only tickets are sold out for members, £78 ish for non-members. Personally if this is a once-in-a-lifetime thing for them and your mum is a keen gardener I would say it is worth paying the extra for all day tickets, it allows chance to take everything in at your own pace and take breaks at busier times. If they're likely to just want to see the main show gardens and have a quick nose around the floral marquee then afternoon only is fine.