How were volunteer fighters who travelled to Spain in order to fight on either side of the civil war treated? Did the Spanish fighters feel respect or disdain when interacting with foreign fighters?
Similarly, how would a British guy going to Spain to fight be treated by his family and friends? Are there any biographies that detail this?
To the main question:
Did the Spanish fighters feel respect or disdain when interacting with foreign fighters?
To generalize to a large degree, they were welcomed warmly. The International Brigades also fought fiercely, so they quickly gained a positive reputation. This reputation was only deepened when the International Brigades took part in repulsing an Italian offensive during the Battle of Guadalajara. When they were disbanded in 1938, their departure was mourned and they were treated as heroes as they left. Other honors were later bestowed on the survivors, but most of those were done within the past 20 years so they aren't applicable for this subreddit.
It should also be noted that the Italian and German contingents that fought on the side of the Nationalists (The Corpo Truppe Volontarie and the Condor Legion respectively) were technically volunteers. In all candor, this was a fiction because these troops were there at the behest of their government. The "volunteer" label was applied so that Germany and Italy could maintain the appearance of following the policy of non-intervention in the Spanish Civil War.
While I understand the suggestions of Orwell's Homage to Catalonia, we should understand the context of that document. It is autobiographical and fairly accurate. However, Orwell's experience should not be extrapolated as a standard for how foreign volunteers were treated in Spain. His experience as a POUM volunteer differed wildly from other communist volunteers and even more from the volunteers that ended up in the International Brigades. As such, I would only suggest reading Orwell (or Hemingway for that matter) after one has a firm grasp on the overall context of the war. Reading any two of the major overview books would suffice (Beevor, Preston, Thomas, or Payne). Zhukov adds more here.
As for how foreign volunteers were treated, other users would likely answer this more completely. I am specifically thinking of the other members of the Spanish Civil War AMA, /u/k1990, /u/Georgy_K_Zhukov, and /u/tobbinator.
How were volunteer fighters who travelled to Spain in order to fight on either side of the civil war treated?
Zhukov's answer on how volunteers reached Spain should answer this part, at least on how they got to Spain in the first place.
As always, followup questions from OP and others are encouraged (though the aforementioned other flaired users might do a better job regarding the volunteers for the Republicans).