What are the guides for lay followers, as to how far they should go to be compassionate? How far should lay followers go, to help others?

There is no denying that we all have innate desire to ease suffering of others, but when it’s done without right understanding it may lead to a higher suffering to the assisted person. On the other had if it’s done with right understanding and with the aim to assist the others to grow in Dharma then it is fruitful.  

As an example, let’s say your friend is grieved for not being able to celebrate New-Year in Paris with his fiancée because he wasn’t able to save up enough money. If you restore in relieving his grievance you could give him money, but this could be adding fuel on fire. A true teacher of the Dharma will correct his view and relieve him from clinging, however, most of us don’t have that kind of ability: we are in a dark, trying to correct our own view, and the virtues we call virtues are but a guess work.

The point that I’m trying to get at is that on some cases the so-called suffering which we trying to prevent on other people could be a wake-up call or a learning curve, and by preventing it we could be hindering the person from realizing the first noble truth.

Sometimes these so called virtues extend even further in the name of compassion and we find ourselves teaching and arguing about Dharma to correct other. There may be many other cases as well. So how should we curb our act not to hurt others and ourselves in the name of virtues?
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.