You are thinking about deprivation of assets.
People also often confuse this with inheritance and general tax legislation.
In terms of the latter you can gift £3k per year without tax repercussions and larger sums that would not attract inheritance tax after 7 years of the gift.
Deprivation of assets is different and does not have a time limit such as the 7 year rule.
It's a somewhat complex area.
If your MIL needed LA funded care in the future as she could not self fund, then if she has given money away in the reasonable knowledge she may need future care provision the the LA can press for the money to be given back under deprivation of assets.
However the onus is on the LA to prove that the primary purpose of the gift was to avoid paying for care or that the gift was unreasonable as the likelihood of needing to fund care now or in the future should have been considered.
As an example, if your MIL had assets of £500k (including property) and gave your son £9k that would be unlikely to be considered DOA - but better she gave him 3k per year re: tax gifting. Essentially whilst it's a reasonable sum, in terms of overall wealth there is lots in the pot left for future care needs.
On the other hand let's say MIL is in social housing, is reliant on state pension and has £50k savings the giving £9k would very likely to be considered DOA.
Age is also a factor. Giving large sums at 60 and in good health is very different than at 80 even in good health simply because it's increasingly likely you will need some form of care/support.
Finally you need to consider "the plan" - some families are in a position they will not ever fund care because they intend to provide it themselves (via multi generational living for example). But this is risky because as capable as some people think they would be at providing elderly care, the reality of it can be very different, especially (and sadly) if conditions like dementia result in extreme challenging behaviours.
Upshot is that you need a lot more information about your MIL's finances (or rather she does) before any substantial gift (in comparison to her overall wealth) is made/ accepted.