Final answer:
The statement is false; testimonials can be provided by clients and former clients. The concept of testimonial injustice relates to the unfair discounting of certain individuals' opinions, not the source of testimonials themselves.
Step-by-step explanation:
The statement that a testimonial cannot be from a client or former client, or a friend/relative of a client or former client is false. Testimonials are often provided by clients or former clients to reflect their satisfaction with a service or product. However, when considering the credibility and ethical use of testimonials, it's important to disclose any relationships that might lead to testimonial injustice, which occurs when the opinions of individuals/groups are unfairly ignored or treated as untrustworthy. Testimonials should be genuine and based on real experiences to maintain their value as a marketing tool.