First thing is pH test the carpet in several places, as well as on the patches. Acid rinse is NOT a de-browning treatment... it is a pH corrector that is intended to minimise the risk of browning after cleaning.
If there are actual patches like stains wicked up, first thing would be to give them a good soak in the right product and then very thoroughly rinse out using a hand tool.
Either buy yourself some prochem browning prescription, or get some acetic acid and dilute accordingly. Spray it all over the carpet, very generously, work it in.. I would use a bonnet (shows remaining soil).. groom the pile and walk away. Do not speed dry, it needs to have a bit of time damp to do its job. If it's alkaline browning that should fix it.
If it is indeed alkaline browning, needless to say this should never happen. Not going to dish out a lecture but bear in mind for the future that you might well have over-done the strength of powerburst and under-done either the rinsing, dose of acid rinse, or a mixture of all three. On a domestic wool carpet I would use it at half strength. You simply don't need it that strong and have to remember your extraction process is only capable of removing a certain percentage of it (this even applies to truckmounts)
If it is just patches wicked back up, completely different scenario... s**t happens, you just have to go back in and soak/rinse them to try and deal with it.
Ask them how long it took to dry. Cellulosic browning from being wet too long is really something that really shouldn't be encountered by a pro
