Help me to help

Hi guys, so I am looking for some help regarding an infected 380 liters beer batch resting in a fermentor at 68 F.I have some friends who have a nanobrewery here in my town and they have asked me for help to try to save this mentioned batch.
I am going to visit the brewery tomorrow afternoon(4 PM Argentina time) to see what this is about.They have told me they find this beer a bit sour, kinda green apple tart taste but there is no traces of any kind of pellicle. It is not a subtle off-flavor , it is a very easy to catch,very noticeable taste.
I have advised them not to discard this batch yet but I am not so sure where should I start to help them.
I was thinking to take a ph read to see how far has beer ph gotten down , then if it is not way too low I am considering to add a pinch of calcium carbonate to a sample to raise ph to 4,2 - 4,5 and see how does beer taste.
I don`t know yet and I think it´s not gonna be easy for me to figure what the cause of the sourness is, maybe it is only acetaldehyde , maybe not. They suspect the culprit is a dirty plate chiller but they are not sure about that either cos there is another batch that is not ìnfected`.
Any help will be appreciated

 

They have mentioned a strong off flavor, weird but you know ...someimes it is not easy to describe.

 

Thanks for the advice, when you say let it set for a little bit, how long does it mean?

 

They haven´t got an alembic.

I was thinking if this batch could not be saved I would suggest them to add my captured wild yeast and wait 10-12 months to see what we get.What do you think?

 

They have enough space and several fermentors but maybe I am wrong thinking about the possibility of continuing with this´ infected´ fermentation because there is no consumer in this town able to enjoy a sour beer with all those funky flavors.

 

Well guys I am updating what I have found today , good news is this batch was not infected(IMHO), I took a sample from fermentor and have found a terrible acetaldehyde aroma and taste.As Hopfenunmalz has advised me I heated it up to 74 F and let it sit for 5 mins, then I could figured acetaldehyde aroma has gone but acetaldehyde taste hasn´t. In the meantime I figured beer was a bit sweet so I started to suspect FG was a bit high, and it was.... 1018 from 1050 OG.Then I asked to the brewmaster why he put fermentor inside the cold chamber for maturation while yeast has not finished its work.He told me that an employee has taken FG read telling him it was 1010. I think that´s not true, I think they have erroneously tried to accellerate the process making a giant mistake.Nevertheless I have advised them to restore the fermentor to fermentation chamber and add more fresh yeast to try to finish fermentation at a little bit higher fermentation temp than the initial(68F) around 72F. I am not so sure how this will turn out but today beer is not very drinkable.

Any thoughts will be appreciated

 

They are using Windsor dry yeast, I have advised them to add new fresh yeast but I haven´t told them to rehydrate it first and let it start a new fermentation in a big starter(say 30 litres 1030 OG wort). Don´t you think it would be better to add an active yeast than a non active one?