"Do not make yourself uneasy, my love. Wherever you and Jane are known you must be respected and valued; and you will not appear to less advantage for having a couple of -- or I may say, three very silly sisters. We shall have no peace at Longbourn if Lydia does not go to Brighton. Let her go, then. Colonel Forster is a sensible man, and will keep her out of any real mischief; and she is luckily too poor to be an object of prey to anybody. At Brighton she will be of less importance even as a common flirt than she has been here. The officers will find women better worth their notice. Let us hope, therefore, that her being there may teach her her own insignificance. At any rate, she cannot grow many degrees worse, without authorizing us to lock her up for the rest of her life." (Chapter 41)

Dressing Chamberlayne in female clothing seems to be a very silly employment for any young lady but especially for a married woman and her husband. Mr. Bennet thinks Col Forster is a sensible man and perhaps he is when it comes to regiments, soldiers and flogging but IMO his participation in this stunt puts a damper on his sense when it comes to the ladies. Col Forster may like a good joke as much as Lydia or he may be just accommodating, wishing to make his young wife and her friends happy but I do wonder how he might keep the respect a commanding officer must maintain when he acts so silly before some of his officers. Lydia is invited to Brighton by her particular friend Mrs. Forster who seems to be the same sort of young silly girl that Lydia is:

But the gloom of Lydia's prospect was shortly cleared away; for she received an invitation from Mrs. Forster, the wife of the Colonel of the regiment, to accompany her to Brighton. This invaluable friend was a very young woman, and very lately married. A resemblance in good humour and good spirits had recommended her and Lydia to each other, and out of their three months' acquaintance they had been intimate two. (Chapter 41)

I cannot find text to show Lydia talked of this incident where her father could have heard of it but Mr. Bennet might have thought better of putting her in the colonel’s care if he had. He could have noticed however that Col Forster married a girl as silly as his youngest daughters. Mr. Bennet, having been fooled himself and found himself married to a very silly woman (Chapter 42) should conclude either Col Forster wanted a silly wife or he was blinded to her character as Mr. Bennet was to Miss Gardiner’s; either reason does not bode well for the colonel’s ability to decipher what Lydia will be up to in a camp full of soldiers at a watering place. Mrs. Forster is certainly no good choice as a chaperone to a silly girl such as Lydia, it is like putting Lydia in charge of herself and Col Forster does not seem to be much better. ;D