If I am proof reading reports should this sentence read...

I think technically the first is correct. As if you split it, as per the second example, the 'However,' doesn't form a complete sentence, it's a clause (I think? I may be wrong!). But tbh I think both make perfect sense, so not sure anyone would pick up on either as 'wrong'. Hopefully someone with more expert knowledge will be along shortly.....

I was told by someone who should know that you should only ever use however after a full stop.

I don't agree - it should start a new clause but doesn't have to start a new sentence.

Shouldn't it be a ; rather than a . ?

Maybe she just meant it shouldn't be used at the end of a sentence? I'd probably use a ;

I'd use a semi colon rather than a comma before however, then it scans better. Full stop doesn't look right somehow - I'm sure however isn't supposed to start a sentence.

To be honest, I would start on a negative and finish on a positive.

WSS.

I agree about swapping the order around too.