Curently from a commonwealth country called Fiji islands. Married and living in germany cause of husband career!.
Wanted my mother to come over and spend 6months with us, went today to book her flight and was told that this will be a problem cause of the requirements of the shcengen visa that she needs to leave the counrtry after her 90days finishes!.
My question is can she leave germany for the uk and re-enter again after a week to used the rest of her 6months stay here in germany?

No, the UK is not a Schengen state and is not covered in a Schengen visa. She willr equire a separate UK visa to enter the UK.

Why not just call or e-mail the local German Customs office for a definitive answer?

The rules are:

- A Fijian may stay in Germany for up to 90 days in any 180. After Day 90, a Fijian must leave the Schengen area for at least 90 days.
- A Fijian must have a separate visa to visit the UK or Ireland. If she's applied for a UK Visitor in Transit visa to change planes in London, it allows only 48 hours in the UK.

It's POSSIBLE there might be special rules if you're married to someone in the British Army and stationed in Germany: check with your base staff if that's the case. Otherwise, the only way a Fijian can stay in Germany for more than 90 days is by applying for a long-stay visa. This is not easy.

I believe sam86 was under the impression you were asking if she had the right to enter the UK, not whether going to the UK, for example, resets the 90day clock in Germany.

flanneruk has given you good info, which answers your simple question. Can she leave and come back for another 90d. That answer is NO, as the rule is 90days in any 180day period.

I'm sorry, I misunderstood the question. Flanneruk is correct.

thanks to all of you who have answered my questions it was very helpfull!