Yeah, when I read a lot of tech blog articles about mobile phones or Wi-fi gear I can't help but think of those timber and stud-wall houses so common in the US, in contrast to Cotswold limestone or Cornish granite homes.
If your mum has decent wired broadband, a pico-cell might be a solution, but network operators do love placing hurdles in the way (for example, EE insist that you get your mobile handset from them with all their bloatware, and even then only a Galaxy S or iPhone).
If I suspect a friend is at home beyond cell coverage I use WhatsApp to invite them down the pub. If I think he's in the pub, I use SMS. It works for me, but it's 2017 and I really shouldn't have to think about it. The good news is that his local has the best pub quiz in the city - due in part to the lack of data signal.