That's a very seisible queston, but would require a complex answer, depending very much on many, many details of the nature and history of the person's disorder, response to treatment, and current problems. SO in the first place you really need to discuss this in detail with whatever psychiatrist is treating the person, who will know the relevant details and should be able to advise best.
Also, consider contacting he Anxiety-Depression Support Group, SADAG, number on this page, as theyre may be a group within reach of you, where you, and the person with Bip;olar Disorder you are asking about, could get useful advice from other people with personal experience of managing the problems