On the contrary. The Tories are doomed already barring some miracle, given their impotence on dealing with the EU or protecting borders. Getting Boris back could only help make the situation less fraught for them, as the alternatives seem incapable or unwilling to stand up for the nation.
They are still in danger of having their core vote split by Reform anyway. But it remains to be seen if Labour can persuade former SNP voters to vote for them instead. May well get a hung parliament next time, and we know how useless they can be.