a magic nightblade is the easiest character to play a thief on, because they have shadowy disguise which they can cast almost indefinatley & a skill which when slotted & they are sneaking or invisible increases their movement speen
however if your partner is drawn to playing a dot based magic charcacter they may also want to consider a magic dragon knight as they have a wider variety of dot based skills to choose from
regarding being a crafter, a thief & damage dealer it is possible, however they will need to do a lot of skill point hunting (just a heads up)
My boyfriend is making a character who is going to be a 4-skill crafter and then he's going to have thief skills on it. He wishes to add in destruction staff. He wants to know if it would be better to be a nightblade or a sorcerer; he is leaning toward nightblade for the DOTs. Is there any reason he should pick sorc over nightblade (skill points, synergy, etc.)?