You or your partner may wish to handle such things differently on different occasions. be prepared to talk and keep talking about stammering with your partner. What you can do is to listen and acknowledge that it is important to your partner. The more you listen to your partner the more you will understand as time goes on. By doing so you will also be 'modelling' for other people how they can behave. Other provision is available through Adult Education or on a private basis.
Some therapy is offered intensively, otherwise e.g. once a week.
Non-speech therapists also offer courses for people who stammer, often people who themselves grew up stammering. There are also self-help groups around the country. The British Stammering Association has information about the range of help available across the country. Successful therapy for your partner may mean that they are stammering more but feeling happier about it. You may wish to be involved in your partner's therapy.