If you offer advice, then it is up to the person to take or not take that advice! if they do, then there should not be any problem, it should not make any difference if you are a professional skipper, as if you are not employed (ie paid) by them, then you have no liabilities.
I am sure a legal eagle would correct me, but from what I remember about contract law, is that you need something of value to be exchanged between the parties (or the promise of value) to constitute a contract. Offering to lend a hand is not a contract, as nothing is implied or given in return.
Law of Tort is different, but I would not have though it applied here, or in many cases.
It is so sad that many people think of "what happens if i get sued" before offering help.
A search might show that British courts deal with very little in the way good samaritans getting sued for helping, as opposed to the American courts which seem to be reporting such cases often.
If i see someone who needs a hand, I offer, If they decline, no problems I go on my way, often they are very grateful for the help. I have towed people out of floods, out of ditches when its been snowy, or broken down, I dont worry about getting sued because i dont think it is a realistic probability.