Hello, this is Jim and I am a dual-qualified lawyer (UK and Republic of Ireland) and happy to help you today.
She can do yes, she can claim against his pension - ideally they would have obtained an order for judicial separation or divorce and had either a clean break order (with no assets to split) or a consent order drawn up and approved by the court to record the settlement.
She may not be entitled to the pension though - the court takes in to account a number of factors when it considers a settlement.
If your partner disagrees with their ex and cannot come to an agreement, your partner can ask the local family court to assess a split of the finances and also to deal with the property including any pensions.
Assuming your partner eventually agrees to a settlement then he can have a consent order drawn up to bind the parties to the agreed terms.
The court will consider the factors contained within Section 25 of the Matrimonial Causes Act 1973 when it assesses a financial settlement. Therefore their ages, length of marriage, any children under 18, their earnings and earning potential (same for your partner's ex), the assets including the former matrimonial home and pensions if applicable, any debts, financial and non-financial contributions to the marriage, their standard of living enjoyed before the marriage breakdown, their financial needs in future and your housing needs, and so on.
The starting point is 50% but if there is a disparity in earnings or your partner has paid more in the way of mortgage/rent, etc, he can ask for more than 50% of the former matrimonial home. It would be worth speaking with her to see if she would agree to any split without having to involve the court.
If nothing can be agreed then your partner would need to issue financial remedy proceedings once decree nisi comes through (if he starts divorce proceedings), or obtain a decree of judicial separation, so he asks the court to become involved in a financial settlement as well as the divorce.
Before he involves the court with the finances he will need to contact a local mediator which is compulsory before he asks the court to intervene in the financial aspect. He can find a local mediator here :
Assuming mediation fails or your partner's ex doesn't agree to mediation then hee will need to issue financial remedy proceedings and he would need to complete and send two completed copies of Form A (one copy is attached) to his local family court and pay a fee of £255 payable to HMCTS.
The court will then list a first appointment and give directions for a financial dispute resolution hearing (FDR) where the judge will give their opinion on a likely settlement - which usually prompts a settlement at this stage. The court will ensure the parties make full and frank disclosure of their assets and liabilities to ensure the financial positions of them both are known and to allow the court to make a settlement decision. If the case does not settle, further directions will be given and a final hearing may take place if neither of them can agree to settlement terms.
To apply for a judicial separation, the site is here: https://www.gov.uk/legal-separation
To apply for a divorce, the site is here: https://www.gov.uk/divorce
And for information on a financial order: https://www.gov.uk/money-property-when-relationship-ends/apply-for-a-financial-order
As it stands though, the property and the pension are assets which can be claimed against.
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.