You have two approaches which work both for problem solving and short technical questions:
The first one is used by your boss: don't provide any help in order to test how the person behave in a stressful context. It is a perfectly valid approach, and may give some hints about the person. After all, once you hire this person, she will not be able to receive constant help from all her colleagues.
The second one is to provide hints and support. The level of support doesn't matter too much; the only thing which matters is that the more help you provide to the person, the less you have to value her success.
Personally, I believe that you should take enough time to both be sure that the person is unable to solve a problem by her own and make the person feel that she's unable to solve it without help. But then, you may provide progressive help until you tell to the person the answer itself.
‒ Can you tell me how do you create read only properties in C#, i.e. properties with a value which can be initialized only within a constructor and cannot be changed later?
‒ Of course. I just use the keyword
‒ Are you sure? Can you explain me the difference between a property and a field?
‒ Hm. A property is... you see... get and set...
‒ Ok. So a field is a variable declared inside a class or a struct and valid within the class/struct scope, while a property is like a field, but also provides a mechanism to read, write or compute a value. Now what about
readonly? Is it used with properties?
‒ I believe that it's used only for fields...
‒ Right. So what about the properties?
‒ They cannot be read only.
‒ Are you sure? What about the properties which have only getters?
‒ They are read only.
‒ Does it mean that their value will always remain the same?
‒ No, not really. The fact that you have a property with a getter doesn't mean that its value doesn't change during the lifespan of the instance of the class. If the getter refers to a field which is incremented each time you access the property, the returned value will continuously increase.
‒ So? Do you have an idea of a way you can implement a property with a value which never changes?
‒ Well, you can use a readonly backing field. Do you know what is a backing field?
Giving the answer is a good idea in all cases. There were several cases when the interviewee commented my answer in an interesting way, showing that even if he was unable to answer to the question in a first place, he still knows related things.
Also, by just asking a question with no further help, you are not having too much information about the person, aside the fact that she knows or doesn't know the answer. Providing progressive help may allow you to see how the person is thinking about a problem.
It may also show other things the person doesn't know. Take the example above: if I would stop at the first reply, I wouldn't have known that the person cannot explain the difference between a field and a property or that she doesn't know what a backing field is.
If the person answers immediately, it's fine. If she needs some assistance, there is nothing wrong with this. If you end up answering the question yourself, it's a bad sign and hopefully the interviewee will be able to answer the other ones.