As you personally didn't witness your friend's dh walking hand in hand with another woman, or observe their body language, I would urge you to use tact and diplomacy when telling your friend what your dp saw.
As part of one of your usual conversations, I'd say 'oh, by the way, X (name of your dp) saw Y (name of her dh) in town earlier/the other day - is your dm/mil/sis/sil staying with you?
If she says no, or asks why you want to know, simply say that X mentioned that Y was walking along with a woman that X didn't recognise so naturally you thought that one of their relatives was visiting - DO NOT MENTION that Y was seen holding hands with the woman.
A quick way to ruin a friendship is to rush in with suspicions/wild accusations about friend's dh, and in this way you won't risk putting your foot in it if there is perfectly innocent explanation.
If there is no innocent explanation, you can be sure that she'll mention the sighting to her dh which will put him on notice if he's doing anything underhand, and no doubt she'll be able to gauge from his reaction if 'something's going on'.
If she suspects that her dh is being underhanded she's likely to confide in you, at which point you can add the 'holding hands' detail without embellishing the tale.