She doesn't need to claim beneficial interest, she may be deemed to have it. According to tax guidelines anyone with lifelong access to the property is deemed to have a beneficial interest.
Anyway, you said and I quote: "If he was over 18 she categorically would not have been in any trouble."
That statement is absolutely incorrect. "if he was over 18 she MAY NOT have been in any trouble" would be accurate.
It depends upon whether the HMRC determined that you have a beneficial interest in the property, and I suggest that someone declaring that (a) it is her main residence and (b) that she will be living there - even temporarily - to look after her son - would stuggle to get away with it.
SDLTM09815 - SDLT - higher rates for additional dwellings: interests treated as owned by an individual, trusts, children [including children subject to the Mental Health Acts]
Trusts
Where a dwelling is owned by another person subject to a trust which gives an individual a right to occupy the dwelling for life or the right to the income earned in respect of the dwelling, that individual is treated as owning the interest.
www.gov.uk
But either way, the reality is more nuanced than you seem willing to accept.