First assessment day is purely written tests. Most of them are based on typical "IQ" testing questions. ie. if this cog goes this way which way will cog number five go etc.
There is a lot of arithmatic which can be quite tricky especially when you know you have limited time available. Best tip here is to forget you are surrounded by people and to relax and concentrate VERY hard.
Most people turn up at the initial assessment looking very smart in suits or jacket and tie. The examiners won't know you from Adam so I wouldn't get too worked up over your appearance as long as you are smartly "casual".
You will hear lots of people talking about how good they are and sucking up to the examiners at coffee breaks; don't bother cos if they could hear themselves they would wish the floor would swallow them up.
Stage two is slightly more taxing where you have computer co-ordination excercises which are not as easy as they look. ie fly the plane through the series of hoops, the more central you are the more points you score.
The other one I remember is the good old "is this shape shown below in the pattern on the screen". More difficult than it sounds, if you dont know then guess and move on to another. You'll have a 50% chance of being right and it saves wasting precious time as the next one might be obvious.
The third has just come to me and it is a series of marshallers silhouettes with batons. Some are upside down, some back to front and some are both. You are asked to very quickly say (via the keyboard) which hand the marshaller is holding up. Dont laugh cos this is the hardest one of all (I thought anyway).
Hope this helps and tell your mate good luck.