I can't see him employing another firm too.
But he may have done.

The only thing is that I ought to have found either their bills or the receipts for them among those papers--except that my late client does not appear to have taken the trouble to keep many receipts." "The thing is that I've learnt that Lord Loudwater had a furious quarrel with some unknown woman between eleven and a quarter-past on the night of his death, and I want to find her.

You can see how important it is.

It may be that she stabbed him, or it may be that she provided him with the motive to commit suicide--not that that seems likely.

But you can't tell: she might have been able to threaten him with some exposure.