In the context of an undergraduate class focused on proof-writing, a small change in what is written donw can make a big difference to the meaning. Are there any guidelines in use on how to mark such work for students with dyslexia (or similar), who are prone to such errors?
I work in disability services as an advisor. Your campus disability services office can certainly help you in terms of talking through how to assess and assign a grade. But, as user6251 said, this needs to be in the letter of accommodation, so the student should register with the disability services office. If the office on your campus has never worked with a case like this, there are ample resources available and they can find out how other universities have dealt with the same issue. AHEAD (Association on Higher Education and Disability) may have some resources on their website as well. Recommendations for specific accommodations should come from the student's clinician (usually the psychologist who diagnosed the dyscalculia).
I doubt that any such marking guidelines specialized for dyslexic students at the college level are likely to exist. (I'm not on expert on learning disability issues, but due to the dearth of answers here, I'm going to take a stab at it.)
At the secondary school level in the U.S., it is required by law that students have an Individualized Education Program (IEP), and this can in fact result in different grading criteria for different students based on their ability level. See the ASCD paper, "Grading Students with Disabilities", for suggestions. Warning: As an anecdote, this may result in the experience of an acquaintance of mine: her middle-school son has an autistic classmate who is effectively nonfunctional, disruptive on a daily basis, and simultaneously carrying the highest grade in the class.
At the college level in the U.S., IEP's do not exist (to my knowledge; definitely not as a matter of law). The emphasis here is generally on a uniform assessment and reporting rubric, with accommodations for doing the work from the accessibility office. In the last few years, the recommendation has come in the form of Universal Design for Learning (UD), possibly including flexible assignments options for different students. (For example, this is now highly recommended by our school's Accessibility Office.) See a primer on UD from the DO-IT foundation at the University of Washington, "Academic Accommodations for Students with Learning Disabilities". Warning: When I've studied UD in the past, I've found the recommendations for math classes to be effectively incoherent, and clearly disconnected from anything a math educator would say (e.g., let students talk to each other during tests; have another instructor come in and write formulas on the board during testing; eliminate word problems because that doesn't really count as math). Also, part of the attraction of UD for accessibility specialists is that it reduces their workload by having instructors do it instead.
In summary: For the U.S., individuated grading procedures on the same assignment are sometimes seen at the secondary level and below, but they are generally not a consideration at the college level. However, allowing students to choose from among several possible assignment options may replicate that effect.
This is certainly a question for the Disability Services Office at your college or University. They will have any existing policy for your school.
In my experience (at 2 different schools), I recieve a letter from Disability Services detailing the accommodations a student is entitled to. I can't say that I've ever knowingly taught a student with dyslexia, because it is none of my business what the disability is and the accomodation letter doesn't (and shouldn't) say. I can say that I've never recieved a letter asking me to grade something differently. The most common accomodations I see are extended time on assignments and notetakers.
I have had a few conversations with Disability services offices before about students, and in those instances the Disability Services people told me specifically not to go beyond the scope of the official accomodation letters (i.e. don't grade any differently).
If a student has told you (the instructor) that they have a disability, but they haven't gone to Disability Services, I would direct them to that office as soon as possible.
(This answer is based on experiences at American Universities.)