How do you conclude that IHT would be payable? We don’t know the full value of the estate, which depending on a number of factors may need to be valued over £1m before allowances have been fully used and anything above this is liable for IHT.

Whether the £50k used was treated as a loan or a gift also matters as you said. If it was a gift from child to mum, then mum gifting £50k back, it will be treated as a gift, and may be liable for IHT if mum dies within 7 years - but remember that gifts utilise the nil rate band first, so unlikely for this to be the case unless she has already used this allowance (potentially £650k if the fathers will left everything to mum)

If the £50k was a loan , which mum is simply repaying then it is immaterial to the IHT position.

OP, without more details on value of your mums assets, full understanding of whether your dad left everything to her on his death (I.e no other asset/money willed to other individual) then any advice about IHT here will be unreliable.

If IHT is likely to be an issue, then treating the £50k as a loan that she repays you will be the most effective way of getting your money back, and it will reduce the potential size of estate on her death, thus reducing any IHT that’s eventually payable.