Plenty of helpful and quite a few misguided posts here. I won't bother writing so much except a few points. Some have been made already but deserve to be reiterated.

The UK is currently looking to reduce immigration. Folks with 10 year old kids are getting visas rejected and being checked out on suspicion of having a sham marriage. The cynical would wonder if the UK have rejection quotas they are asked to meet any way they can. The UK is seeing a massive increase in families whereby the kids only see daddy on skype because daddy is foreign. Immigration targets are paramount, human right to family life secondary. They wouldn't stand a chance proving the history necessary. I am a Brit married to someone from outside of Europe and we literally can't live in the UK. I had to leave my home to continue the relationship. I know many other couples who are doing long distance long term or stuck in the other partner's country just to stay together. Things are strict right now.

Also the money. There are super strict financial requirements. Half of the UK literally can't even afford to sponsor a foreign partner. He may well be in that 50% in which case good for him. If he's disabled and in receipt of certain benefits this won't apply. Still a shit ton of paperwork though, like you wouldn't imagine.

You can't marry on a tourist visa. Even if she keeps her mouth shut at the border they wouldn't let her give notice at the register office. Folks have done this in the past but as I said the UK is cracking down hard. She'd need a fiancé visa.

A visa is not citizenship. She'd be lucky to get a visa at all, but citizenship would take years.