Boswell met Mollison during his stays with Stewart in 1763 and 1764. On January 11, 1764, he played cards with Mollison and his wife for 7 hours, after which he made a note to "make a firm resolution, a promise, never to play but when necessary, as 'tis low and unworthy." 2
Note 1: A son was born to a John Mollison and Sarah Mills at Rotterdam in June, 1766, which is my reason for naming his wife Sarah Mills, although her last name didn't appear in Boswell in Holland 1763-64.
Note 2: One may wonder when it is actually necessary to play cards - presumably he means, that on certain social occassions it may be better to play.