Disclaimer: the following is not legal advice in any juristiction
In the situation described, common sense need to prevail.
You are monitoring SAR frequencies, and for that: the SAR agencies are as well. If you hear a distress message to a SAR agency you should relay this in a legal manner: e.g. you contact the SAR agency by cell phone, or on a frequency you are licensed to use. Relay what you have heard, and on what frequency you have heard it.
You need to keep monitoring the frequency, to establish if a SAR agency tries to contact the distressed party. Please keep in mind that while you may hear the distressed party, you may not hear the SAR agency.
If the distress signal/message continues, and you have established that no official SAR agency is responding to the signal, you should answer the distress call; identify yourself, get as much information as possible, such as nature of the distress, location, number of people in the party, and any other information. Keep this communications as short as possible, as you are using a frequency you are not licensed to use, and you do not want to interfere with official communications.
Once again, relay this information to the SAR agency.
You are now the "gateway/relay" for communications and you need to follow the instructions of the SAR agency.
Keep monitoring the frequency, and once in a while... say every 10 minutes, contact the distressed party to ensure them you are in contact with a SAR agency. But ONLY if the SAR agency has not established contact themselves.
While doing this, in parrallel, you need to establish if you, or someone in your party can provide assistance to the distressed party. However; make absolutely sure that you are physically able to do so, and not putting yourself or others in harms way !! You do not want to become a second distressed party !
When you approach this in a proffessional manner, keep communications to a minimum on unlicensed frequencies, and follow the instructions of the SAR agency, you will not find a judge which will prosecute you for using unlicensed frequencies.