If a director is honest and does what is right for the company / shareholders, and complies with the law - then they will never be liable personally for debt (it is a limited company - which means limited liability). Liability is limited to the paid up value of the shares.
The only exception would be if for example there is a loan which requires director's to take personal liability but that would be a separate form to sign if that ever came up.
The director's job is to comply with the law and then to run the business for and on behalf of the shareholders - usually that means making profits!
There can be several types of director - in a large business it can be just a job title - you direct others / a board director - you also sit on the board / a company director - you are registered as a director at companies house...
in a small business there is usually only one type of director.
if he sits on the board without being a director, he will probably be considered by Companies House to be a shadow director or POSC (person of significant control) and have to be registered as such - there is little difference in liability or in legal terms between being a director or POSC.
there is unlikely to be much negative outcome in being a director and it is usually seen as an honour / promotion. As above - there is little liability if he is honest and does what is right... If there are dodgy decisions being made by the board which might suck him in, then he declares a contrary position in the meeting minutes, or resigns - no risk. (I suspect though he already knows how the business is run and that shouldn't be an issue).
I would though see if he can get shares if at all possible as that is where you make money in a business - so may be worth a discussion to see if there is any option for that now or in the future...
but don't fear it - it is a compliment and worth doing...
(my background - lost count of how many directorships I have had in my businesses and others - and act as a management consultant advising companies and board members...)