can now signup to fibre broadband.
With which company? That affects what services may or may not be available. If it's Openreach-based then they could take service from BT, EE, Sky, Zen or Vodafone and have an included voice service where they just plug in the existing handset into a router. There's a single bill, and a single point of contact if there's any issue.
Other fibre providers may or may not offer a voice service - most don't.
she doesn't want to be without a landline.
Does that mean "I want people to be able to call me on my landline number?" Or does it mean "I want to pick up a physical handset even if my mobile phone is misplaced / battery is dead"? In the latter case, does she want a wired handset or a DECT-style wireless handset?
As for the option of broadband and VOIP separately: it's worth considering. and I don't think adds significant complexity after a one-time setup. You can get the VOIP service for about £1.20 per month (AAISP) for incoming calls. If they already have mobile phones then I'd suggest using those for outgoing calls; you can get packages with unlimited calls for under £5 per month (e.g. Lebara). If mobile phone coverage is poor, then Wifi Calling might be the solution, if supported by the provider.
If they do already have mobile phones, with reasonably modern Android or iOS, then I'd suggest buying Acrobits Softphone (one-off cost of £6), and ditching the old handsets entirely. Then they can receive "landline" calls wherever they are.
Another option is simply to divert the old landline number to a mobile - services like numberpeople.co.uk - although I don't know if they're able to forward to two mobile numbers simultaneously, so that the first one to pick up answers.