I have not read Folland et al, 2005, but perhaps what that study would suggest is that your friend could do both dead hangs and bent-arm hangs. But, doing dead hangs while trying to do a chin-up would increase the angle over days and weeks as well. It's not clear from your statement whether your friend hopes to simply hang there and then one day be able to do a full chin-up, or he plans from the outset to keep trying to do a chin-up while hanging. The study "Do isometric, isotonic and/or isokinetic strength trainings produce different strength outcomes?" by Lee et al is relevant. They write:
After training, muscle lean muscle mass increased in isometric (+3.1%,
p < 0.01) and isotonic groups (+3.9%, p < 0.01); only the isokinetic
group showed a significant improvement in the triple-hop-distance test
(4.84%, p < 0.01). ... Kovaleski et al. (1995) found greater muscle
torque increase in isotonic than in isokinetic exercises, while Chen
et al. (2015) showed better results with isokinetic exercises. Remaud
et al. (2010) found similar significant strength gains in both
exercises; however, these discrepancies could be due to high speeds of
isokinetic training (150 and 180°/s) and a relatively low isotonic
training load (40% of maximal voluntary isometric torque at 70°);
probably more pronounced changes could be observed with a more intense
program.
Although the number of studies on different types of muscle
contractions is relatively large, few have compared isometric to
dynamic exercises (Folland et al., 2005, Malas et al., 2013). One
limitation of isometric training is that the strength increases are
specific to the angle used. To overcome this problem,
strength-training series were proposed to be done at different joint
angles (Folland et al., 2005); the authors found that isometric and
dynamic exercises showed similar isokinetic strength gain, but the
gains in isometric strength were significantly greater on the
isometrically trained lower limb.
