You can ask them to add a number 1-4 after the Pinyin of each word to indicate its tone. For example, let them type 朋友 as 'peng2 you3' instead of 'pengyou'. Actually even native Chinese speakers can not read a toneless Pinyin sentence quickly, though we can figure it out by experience.

As we all know there are many homophones in the English language, for non-english speakers we might pose the same question "how can I understand which word they are trying to use?" Did you bring her flowers? Did you bring her flour? Although they sound similar in daily conversation, but I believe I won't make you feel confused because they are used in different contexts, the same thing goes for pinyin, we won't misunderstand what you are trying to say normally, even they are the same Pinyin with the same tone. If you bring your girlfriend flour, I would follow up with a question "Is she working in a bakery?" to avoid misunderstanding, right?

I think only native speakers are capable of understanding. Only limited combinations of tones for the syllables make sense, but it orders the sensitivity to the language.

Not all the combinations make sense. Like the example Chiffony gives, 朋友，peng2you3, if you pronounce it like peng1you1, peng1you2, peng1you4, etc. they don't make sense at all. Chinese does not have words pronounced like them. Sometimes, there will be several combinations that make sense. But if you put them into a whole sentence, then usually only one of them can make sense according to other words in the sentence. It's quite easy for native speakers to tell which one is the right one. But I guess it might be very hard for foreigners. So ask your friends to write down the tones at the same time or just try to read them in different tones and see if any of them make sense to you.

一般来说在不标单调的情况下，读个一两遍就能知道每个字的调值。这个主要是根据语义判断的。不过这个对于把普通话作外语学习者来说确实有会有点障碍，不过多练习就会好了。但普通话相对来说已经很好判断了，它只有四个声调，粤语有九个声调。汉语并不是很好学的语言