Friction Dermatitis: This is the most likely diagnosis because the scenario describes skin rubbing against each other, which is a classic cause of friction dermatitis. This condition occurs due to repeated friction, leading to irritation and inflammation of the skin.

Intertrigo: This condition is characterized by inflammation of the skin folds and can occur in areas where skin rubs against each other, such as the inner thighs. It is often caused by friction, moisture, and bacterial or fungal infections.
Chafing: Similar to friction dermatitis, chafing is caused by skin friction and can lead to redness, irritation, and discomfort. It is common in areas like the inner thighs, especially during activities that involve repetitive motion.

Erythrasma: Although less common, erythrasma is a bacterial infection that thrives in warm, moist areas of the body, such as skin folds. It can cause redness, scaling, and itching, and if missed, can lead to more severe infections.
Tinea Cruris (Jock Itch): This is a fungal infection that affects the groin area and can spread to the inner thighs. It is essential to consider this diagnosis to prevent mismanagement and potential complications.

Hailey-Hailey Disease: A rare genetic disorder that affects the skin, causing blisters and erosions in the skin folds, including the inner thighs. It is crucial to consider this in the differential diagnosis, especially if the condition does not respond to typical treatments for friction or intertrigo.
Darier Disease: Another rare genetic disorder that can cause skin lesions and erosions, particularly in areas subject to friction. Although rare, it should be considered in cases where the presentation is atypical or does not improve with standard treatments.