I do wonder if it's the other way round. I think Trinny is pretty steely - I don't buy this whole 'I'm the big sister everyone wants act', this is the woman who not only was able to handle a man known for domestic violence but actually got him to put up the initial funding for her brand before getting shot of him (I know others love her, that's ok).
So I could see Hirons befriending Trinny, because we know Hirons loves a high profile friend. Hirons the big mouth then boasts about the profitability of skin care etc, assuming that Trinny was going to stick with make up. Trinny though is quietly vampiring Hirons' ideas, and as she's got more money, she's able to get a sleek brand to market first, making Hirons' look like the sad copy, even though it was probably in development longer.
Too late, Hirons' realises she's been had (this just shows the margins in skincare must be much higher than makeup if they are all piling into it), but it's her own boastful fault, and the best she can do is try and trash the Trinny microneedling and buy up her daughter's domain name.
Meanwhile, Trinny can laugh all the way to the bank, and has no need to trash Hirons as she's the winner!
Eta, I don't like either of them, so not defending anyone here, they are both as awful as each other.