What are the guides for lay followers, as to how far they should go to be compassionate?
The most important things is to get Panna yourself as much as you can. You can achieve it by either reading Tripitaka or consulting monks about wholesome and unwholesome acts or both. 
Passionate or compassionate about others is generally speaking wholesome act. But it has it own merit as well as harm to helper and helped if improper intention or volition. 
To be specific to your question, 

Donation is good to donor as long as donor have no attachment to things donated and have Metta to donee that he/she will free from sufferings. That is the very minimum requirements to have mind in donor. (There are many more details about making donation to the best, you can consult in Tripitaka and/or a well taught monks). In this case donor won't worry about how donee will use donated material/money. 

For mental or verbal assistance,
Try to make your friend cheerful by encouraging, assisting to understand sufferings are not persisting. Try to do some activities and distracting him/her from thinking negative ideas if you are persuasive enough (you can either read some from Buddha's teachings or other religious teachings and non-religious self-help. The more you read, the more persuasive you are. Words have power). 

For helping other people from clinging, you are absolutely right. No one, except Sammasam Buddha is the best because people with different hobby, desire, volition, perception and attitude need different delicate guidance to right path. We can only recommend some right ways to soothe sufferings. You can recommend him/her to do some meditation/Vipasana. Let him/her find what is bothering by himself/herself. How he/she can understand sufferings and how Buddha's Dhamma can guide him/her right way to end sufferings. There is nothing wrong if you are pushing someone to get to know Buddha's Dhamma as long as you put Metta on the people you are pushing. I read in Buddha's past life, he was pulled from hair to bring in front of Buddha and to get to know prior Buddha and Buddha teachings. But in this age of personal freedom, communities and societies don't go details to true intention of pushing someone hard and is not appreciated.